1. Field of the Invention
The present invention relates to an induction heating stress improvement method (hereinafter abbreviated as “IHSI” method) in a primary loop recirculation piping (hereinafter abbreviated as “PLR” piping) of a nuclear power plant.
2. Description of the Related Art
As an example of conventional IHSI method for a double metal, there is a method disclosed in JP, A 2-282428. In this conventional art, water flows are alternately sprayed into an annular space, and the nozzle outer surface surrounding the annular space is heated to provide a temperature difference to the nozzle wall, whereby residual stress is relaxed.
Conventional inventions, however, have encountered a problem in that they exerted an enhanced cooling effect but have been incapable of verifying a cooling effect when applying IHSI to an actual machine. Particularly in a recirculation inlet nozzle (hereinafter referred to as an “N2 nozzle”) having a narrow annular clearance, it is very difficult to spray a fluid to produce a flow in the annular clearance. Furthermore, the inner surfaces of the piping of actual machines would have various asperities depending on the individual piping although these are merely slight asperities to the extent they present no problem in terms of performance and structure of the machines, and/or the outer surface of the piping may not have a perfectly circular cross-section. As a result, it is difficult to dispose a cooling nozzle apparatus in the reactor in a proper position, thereby causing a possibility that the nozzle may not be directed toward the direction of the annular clearance. Hence, it is necessary to verify, in the actual machine, whether a jet is sufficiently flowing into the annular clearance. That is, it is important to verify a cooling effect with respect to the inner surface of the piping when applying IHSI to the actual machine, and a contrivance is needed to cause the jet to efficiently flow into the narrow annular clearance.
Conventional techniques require a plurality of devices such as a large number of jet nozzles, piping for use in connection, and the like, in order to secure a sufficient cooling effect. In a nuclear power plant, a rationalized apparatus is desired from the viewpoint of reducing radioactive waste to a minimum. If an apparatus which allows a cooling effect to be verified, and a jet to be efficiently flowed into the annular clearance, can be applied then the number of above-described redundant devices can be reduced.
Furthermore, in the PLR piping of a reactor, a flange or an air vent cannot be installed from the viewpoint of ensuring safety and leakage prevention. As such, air could stagnate in a closed stagnation portion such as the nozzle to decontaminate, and might make it impossible for the cooling water to perform cooling when executing IHSI, which entails the removal of air. This is because air is very low in thermal conductivity and heat transfer coefficient, and poses a significant detriment to the execution of IHSI. Such stagnant air cannot be removed by only water flows, which has become a problem associated with the application of IHSI.